It depends which environmental impacts you are talking about. In general, when we talk about any environmental impact, the process that "impacts" the environment is in most cases a chemical process or, better to say, a complex chain of physical and chemical processes. Therefore, to influence it one needs to use chemistry for reducing almost any environmental impact. For example, when you have an oil spill in the sea, physical methods (collecting the oil) are combined with chemical methods (adding the chemicals to create a suspension to avoid the spreading of the oil). Acid spill can be partly neutralized or mobile hazardous substance can be bounded by adding a proper chemical. While producing different things, chemical reactions are used to avoid unnecessary emissions. On the water treatment plants chemical processes are used to clean water to avoid unhealthy emissions into the rivers. From the other hand, i believe that the greatest way the chemistry can reduce the environmental impacts is by avoiding them. As chemistry develops, we learn more about how chemical processes work in the environment and we can try to do things we need without impacting the nature. For example, we need to add fertilizers in the soil to grow enough crops. Fertilizers we used few decades ago were much more dangerous for the environment (e.g. read about DDT), while modern fertilizers are more environmentally friendly as we learned more about how chemistry in soil and plants work.

This is a question that cannot be completely answered in an email because there are so many ways currently and potentially that chemistry can help.A couple ways that came to mind immediately for me is the idea that there are organisms that can be used to eat oil (from spills in the ocean or other water environments) and the use of trees to uptake and store carbon (a chemical reaction within the plant). Hopefully others can chime in and give other examples, because the answer to this is unlimited ways.